Members used in airplanes and power generation turbines include an Ni-base superalloy represented by 718 alloy which is excellent in corrosion resistance and high temperature strength. The crystal grains and the precipitation strengthening phase of the superalloy used in the above-described members of airplanes and power generation turbines are adjusted in size by hot forging and heat treatment. As a result, this superalloy has excellent high temperature strength.
Among these, for example, a turbine disk is a large-sized rotor having a complicated shape. In addition, fatigue strength, among strength properties, is particularly regarded as important. Therefore, in a hot forging step, securement of the shape of a large-sized product and containment of fine crystal grains in the endoplastic surface are required to be achieved by near-net-shape closed die forging. Crystal grains become finer by sufficiently promoting recrystallization at the temperature range which allows for the precipitation of pinning particles. Accordingly, an extraordinarily large forming load is required for balancing both of shape and quality in the closed die forging of a large-sized rotating member. However, realistically, a limit exists in press load capabilities.
Therefore, a lubricant is applied on a forging stock during hot forging. The main effect of a lubricant is the operation of reducing the frictions between a forging stock and a die. This effect is achieved by forming a continuous lubrication coating film on a forging stock while maintaining an optimum viscosity of a lubricant during hot forging. In order to manufacture a large-sized forged product within the range of press load capabilities, the role of a lubricant in reducing a forming load becomes important.
The invention of this hot forging method with a lubricant is disclosed in, for example, JP-A-6-254648 (Patent Literature 1).